The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, an automatic transmission with more speed stages has been developed to enhance fuel economy and drivability of a vehicle. The increase of oil price makes it difficult to enhance fuel consumption of a vehicle.
In this sense, many researches for reducing weight and enhancing fuel economy through downsizing of an engine and for securing drivability and fuel economy through multiple speed stages of automatic transmissions have been developed.
For more shift-stages of an automatic transmission, the number of internal parts is typically increased, which may affect installability, production cost, weight and/or power flow efficiency. In that aspect, less number of parts is desired to enhance fuel efficiency of an automatic transmission having more shift-stages. An eight-speed automatic transmission has been introduced recently and a planetary gear train for an automatic transmission capable of more shift-stages is under development.
However, a conventional eight-speed automatic transmission typically includes three to four planetary gear sets and five to seven control elements (frictional elements), and this may largely affect the weight of the vehicle.
In this regard, disposing planetary gear sets in parallel or employing dog clutches instead of wet-type control elements has been attempted.
However, such an arrangement may not be widely applicable, and using dog clutches may easily deteriorate shift-feel.